Which of the following statements are correct about $\mathrm{Zn}, \mathrm{Cd}$ and $\mathrm{Hg}$ ?

$A$. They exhibit high enthalpy of atomization as the d-subshell is full.

$B$. $\mathrm{Zn}$ and $\mathrm{Cd}$ do not show variable oxidation state while $\mathrm{Hg}$ shows $+\mathrm{I}$ and + $II.$

$C$. Compounds of $\mathrm{Zn}, \mathrm{Cd}$ and $\mathrm{Hg}$ are paramagnetic in nature.

$D$. $\mathrm{Zn}, \mathrm{Cd}$ and $\mathrm{Hg}$ are called soft metals.

Choose the most appropriate from the options given below:

Properties common to the elements manganese, iron, cobalt, nickel and copper include the ready formation by them all of

$(I)$ coloured ions in aqueous solution

$(II)$ oxides of nitrogen are formed on reaction with concentrated $HNO_3$

$(III)$ chlorides of formula $MCl_2$ and $MCl_3$

Manganese steel is used for making railway tracks because

$Fe,$ $Co$ and $Ni$ have valuable catalytic properties in process involving

Although $\mathrm{Cr}^{3+}$ and $\mathrm{Co}^{2+}$ ions have same number of unpaired electrons but the magnetic moment of $\mathrm{Cr}^{3+}$ is $3.87$ $B.M.$ and that of $\mathrm{Co}^{2+}$ is $4.87$ $B.M.$ Why ?

Which of the following compound is coloured ?